The purpose of this paper is to examine the process of interlanguage development in Japanese adult learners of English as a Second Language (ESL) in acquisition-poor environments. A total of 353 subjects were divided into 9 groups based on their level of English proficiency and tested to determine: (1) what types of errors Japanese adult ESL learners make as they acquire the English article system; (2) when these errors appear and disappear in the developmental process of interlanguage; and (3) why such errors especially occur in Japanese adult ESL learners in acquisition-poor environments. The study found that co-occurrence and word-order errors sharply decreased once students reached proficiency level three. Underextension and substitution errors were sharply reduced from levels one to four, and persisted in the interlanguage at level nine. Overextension errors were predominant above level four and persisted in the interlanguage. Substitution errors were sharply reduced from level one to level four and persisted at level nine. These findings have important pedagogical implications regarding the organization of the English-language syllabus in Japanese schools. (Contains 63 references.) (MDM)